1. Field of the Invention
The present invention relates to a level shift circuit and a display device having the same. More particularly, the present invention relates to a level shift circuit converting a level of output signal from a circuit operated with one supply voltage into a level adaptable for a circuit operated with another supply voltage and a display device having the level shift circuit.
2. Description of the Related Art
Conventional technology of integrating a circuit, which is installed in a liquid crystal display (“LCD”) on the same substrate as an LCD substrate, has been developed with the purpose of making compact and inexpensive LCDs with high precision. As a result, the LCD substrate integrating a driving circuit having thin film transistors (“TFTs”) has been known.
Due to the use of TFTs, a peripheral circuit such as a data line driving circuit is partly integrated. Thus, the number of driving large scale integrated (“LSI”) circuits is reduced, and a mounting cost is saved. For the data line driving circuit, a digital-to-analog converter (“DAC”) converting digital display data into analog signals is mounted on the LCD substrate.
An image signal, which is input to the LCD substrate mounting the DAC, is a digital signal. This digital signal is usually generated by a signal processing circuit (also called an external signal processing circuit) installed outside the LCD substrate.
In general, the external signal processing circuit includes a complementary metal oxide semiconductor (“CMOS”) integrated circuit, and it generally has a driving voltage lower than the supply voltage required to drive the TFT integrated circuit. For example, the external signal processing circuit is operated at a supply voltage of 3.3 V, and the TFT integrated circuit requires a supply voltage from 5 V to 10 V in order to drive the LCD substrate at a sufficient speed, or in order to apply a sufficient voltage to liquid crystals. Hence, the TFT circuit is driven by boosting a logic signal of 3.3 V to a range from 5 V to 10 V using a level shift circuit that is integrated on the LCD substrate. However, the conventional inverter type level shift circuits give rise to dispersion when processing high-speed data at the same time because a response speed is dependent on threshold dispersion of the transistor.
Further, many level shift circuits have been suggested, but each level shift circuit makes use of many circuit devices, and thus there is a large circuit area, as well as high power consumption. Moreover, when the TFT with low electron mobility is used, a transient response speed is lowered.